We have demonstrated that CMV/SIV vectors can 1) re-infect CMV+ rhesus macaques (RM), 2) during re-infection, elicit potent and persistent SIV-specific CD4+ and CD8+ T cell responses with a strong "effector memory" (T{EM}) bias, and 3) completely protect ~50% of vaccinated RM from progressive infection after limiting dose rectal challenge with the highly pathogenic SIVmac239 virus. The protection manifested in these RM is distinct from previous vaccines in its abruptness and extent, with protected RM exhibiting a viral burst in plasma of varying size upon initial infection, followed by immediate control to undetectable levels. Although occasional viral blips in plasma are observed in protected RM, these decline with time, and after 1 year, protection is unaffected by CD8+ or CD4+ cell depletion, and extensive tissue analysis with ultrasensitive nested PCR has shown only rare detection of ~ single copy SIV nucleic acid and no viable SIV. Protection correlates with the total SIV-specific CD8+ T{EM} generated during the vaccine phase, and occurs without an anamnestic response. These data indicate a novel pattern of protection consistent with very early control, likely taking place at the site of viral entry and/or early sites of viral replication and amplification, and involving tissue-resident CD8+ T{EM-} Thus, CMV vectors and the "T{EM}" vaccine concept offer a new and powerful approach to HIV/AIDS vaccine development. In this project, our major goals are to delineate the mechanisms responsible for this unique protection, and determine the quantitative and/or qualitative determinants of protection vs. non-protection. First (S.A. 1), in collaboration with Project 1, we will use a serial necropsy approach to define the spatiotemporal progression of SIV infection after mucosal challenge of unvaccinated RM, both the trajectory of viral infection from the portal of entry to systemic infection, and the development of the anti-SIV immune response. Then (S.A. 2), with this baseline established, we will determine when and where the high frequency, tissue-based SIV-specific T cell responses elicited by RhCMV/SIV vectors intercept infection after mucosal challenge, delineate the function of these cells at this intercept, and determine the functional requirements for stringent viral control ~ in comparison to responses elicited by replication-deficient adenovirus and poxvirus vectors (Project 1) and live attenuated SIV vaccines (Project 3). Finally (S.A. 3), we will determine the extent of latent and/or active infection in short-term vs. long-term RhCMV/SIV vector protected RM and the activity of SIV-specific T cells in these RM, so as to determine the mechanism of long-term aviremic control, and to explore the possibility of eventual functional clearance of the infection.